The City of Biddeford retained Weston & Sampson to provide consulting engineering services for the repair of failing retaining walls along sections of the Saco River.
The wall system in the project area is approximately 33 feet in height, and has historically consisted of stacked, non-mortared granite block wall downstream, and stacked, non-mortared granite rubble walls upstream of the project area, near the historic Boiler House building and the approximately 250-foot-tall smokestack.
In 2006, an approximately 40-foot-wide section of the granite rubble wall failed and collapsed into the Saco River. The failed section of wall was replaced in 2011 with a Mechanically Stabilized Earth (MSE) wall, in addition to the construction of an overhang deck structure, designed by another consultant. On October 31, 2021, during a period of significant rainfall, the Biddeford area received approximately 6.5-inches of rain. An approximately 80-foot-wide section of the rubble wall and the wall/overhang structure failed, leaving a steep, sloughed soil slope along the failed wall section. Failure of the retaining wall caused significant damage to the adjacent MSE wall and overhang structure, including a loss of backfill and geogrid support behind the MSE wall, displacement and reduced support of wall blocks that were structurally connected to the failed wall, and outward displacement of the MSE wall support base.
Weston & Sampson retained Doucet Survey, LLC to complete a topographic survey of the project area and surrounding features. Once completed, we used the survey to generate an existing conditions plan for the city to illustrate alternatives for restoring the failed retaining wall. In addition, Weston & Sampson completed an exploration program to collect information on the bedrock and soil conditions below and behind the wall failure area necessary to evaluate the geotechnical and environmental considerations and appropriate wall replacement alternatives. Based on the level of repair necessary and unknowns of the full extent of damage, Weston & Sampson determined that replacing or repairing the wall is not the practical or reliable solution and recommended replacement with a new retaining wall that extends from the bulged area downstream to approximately 25 feet beyond the upstream end of the Boiler House building.
LOCATION: Biddeford, ME